Metric Size of Bolt Heads

Written by andrea stein

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Bolt and pitch are two factors to consider when determining bolt head metric size. (bolt image by Petr Gnuskin from Fotolia.com)

Bolts refer to fasteners that are characterised by a spiral, or helical, ridge, known as an external thread. The diameter of these threads helps determine the metric size of bolt heads, or the section at the end of the bolt that enables it to be driven or turned.

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ISO Standards

The ISO, or International Organization for Standardization, determines the international standards for the metric size of bolt heads. These standards are defined by the bolt threads, which are measured based on diameter and pitch.

Bolt Diameter

Bolt thread diameter, or the measurement defined by the end points of a straight line that passes through the centre of the circular area of the thread, is designated by ISO by the letter M. The diameter is measured in millimetres; therefore, an 8-millimeter bolt thread would be classified as M8.

Bolt Pitch

Bolt pitch refers to the distance between turns of the screw thread and is measured in TPI (threads per inch), or how many threads occur per inch of screw length. The most common pitch value for each bolt diameter is called the coarse pitch. The standard coarse pitch for an M8 bolt is 1.25mm, and is expressed via a multiplication sign, such as M8 x 1.25. Any variants, or extra millimetres measured in the pitch, are considered fine pitches.